Reaction yield detected magnetic resonance and magnetic field effect studies of radical pairs containing electronically excited organic radicals

Abstract

Observations are reported of a previously uninvestigated type of spin correlated radical pair in which one radical is in an electronically excited state. The species was produced by two-photon absorption from a single laser flash, and the fluorescence from the excited radical was detected. The experiments performed involved studying the effects of an applied static magnetic field (MARY), and the effects of a resonant microwave field in a high applied static field (RYDMR). Both have been used in a time resolved fashion, besides their conventional one, and pulsed RYDMR experiments have also been performed. A series of pairs containing radicals derived from diphenylmethyl have been studied in glycerol solution, and in SDS and Triton X-100 micelles. The field effects in the MARY experiments varied considerably from one system to another, and failed to correlate directly with either the magnetic properties of the radicals or the molecular dynamics. It is suggested that the nature of the reaction exit channel plays an important part in determining the overall reactivity. The systems exhibited B 1/2 values much larger than expected, which is shown to be consistent with a contribution to the spin dynamics from relaxation induced by modulation of the electron exchange interaction. A striking feature of the RYDMR experiments performed on micellized systems, rather than in glycerol solution, was an unexpected inability to induce state locking at high microwave field strengths. This could not be rationalized using any previously suggested mechanisms, without these implying large increases in the widths of the RYDMR B 0 spectra, which were not observed. It is suggested, tentatively, that this originates in an electronic energy exchange process within the geminate radical pair; this is consistent also with a lack of a low-field minimum in the MARY curves from the micellized systems. An attempt has been made to simulate the RYDMR B 0 and B 1 spectra using a stochastic Liouville equation approach.